This invention relates to a paper carrier material for photography which is coated with a synthetic resin and which can be dried by using microwave radiation.
It is known that paper strips can be dried with microwave radiation and that photographic papers can also be dried using microwaves after exposure and processing in the usual photographic baths. This technique is used particularly when rapid heating of the material being dried is desirable. An advantage of microwave drying is that energy absorption is directly proportional to the water content of the paper.
It is known that photographic papers coated with polyethylene can be treated with salts, e.g. the sodium salt of naphthalene-sulphonic acid or Na.sub.2 SO.sub.4 (British Pat. Nos. 1,019,664 and 1,346,960). This treatment is for the single purpose of preventing a critical accumulation of electrostatic charge in high-speed processing machines, because the accumulated charge can interfere in the transport of the paper or can cause undesirable exposure of adjacent photographic layers by the formation of sparks.
Chu et al. (U.S. Pat. No. 3,253,922) discloses photographic paper coated with polyethylene which has been treated to reduce the formation of static charges. This is accomplished by incorporating in the paper base an antistatic agent, e.g., sodium sulfate, salts of organic compounds, and organic antistatic agents, including trialkanolamine alkanoates and polyalkylane polyamine derivatives.
Seel (U.S. Pat. No. 1,434,453) teaches the use of an inert metal salt and a hydroscopic organic compound in a nitrocellulose support for photographic film.
Inclusion of a deliquescent salt, e.g., CaCl.sub.2, MgCl.sub.2, or LiCl, in a resin-coated paper, whereby the water content of the paper substrate is held at 6-10% during manufacture and the paper is prevented from curling, is disclosed in German Offenlegungsschrift No. 2,235,032 (Jan. 25, 1973).
Verburg (U.S. Pat. No. 3,769,020) provides a photographic material having a backing layer of polyolefin on both sides, the rear surface of one polyolefin layer carrying an antistatic layer, e.g., polystyrene sulfonic acid in a resin.
Dippel et al. (U.S. Pat. No. 2,588,218) treat dry photographic material of regenerated cellulose by heating in a high frequency electric field and passing a stream of gas over the material.
Minagawa (U.S. Pat. No. 3,884,692) teaches the application of a coating of colloidal alumina to the back of a polyolefin laminated paper support for light-sensitive photographic material to prevent the paper from blistering during drying by microwave heating.
The advantages of microwave drying have not heretofore been to any great extent for drying photographic papers coated with a synthetic resin, because it is not practical to raise the water content of photographic papers too high. For example, when water content attains equilibrium at 70% relative humidity or more, increase growth of fungus and bacteria can be observed and results in paper having a mottled appearance and being partially destroyed. In addition, the influence of dampness on photographic layers containing silver salts is apparent even through layers of a synthetic resin. The water content of the paper core influences adhesion, hardening and the sensitometric properties of the photographic layers. When such layers are to be used in color photography, the presence of this water manifests itself in the form of undesirable color changes. Therefore, in base papers for photographic purposes the water content is carefully adjusted to a constant value, preferably corresponding to an equilibrium water content at 50% relative atmospheric humidity or less. Only in exceptional cases are equilibrium humidities which correspond to 55% or a maximum of 60% relative humidity preferable.
The energy absorption observed when microwaves are used is influenced not only by the water content, but to a lesser degree by components in the paper. The amount of these additional materials and their dielectric constants have the most important influence. If the water content of the paper is constant, the effect of these additives at low concentration is very small, since their dielectric constants are generally very much smaller than that of the water. Only mineral fillers are an exception to this rule. However, it is not advantageous to use these fillers because of the irregular energy absorption caused by their distribution in the paper and their microscopic particle structure.